Fits and Tolerances - City, University of Londonra600/ME1105/Lectures/ME1110-07.pdf · To learn about fits and tolerances ... 1 Nominal Size– a general size, ... Actual Size –

Ahmed Kovacevic, City University London

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Fits and Tolerances

Prof Ahmed Kovacevic

School of Engineering and Mathematical SciencesRoom C130, Phone: 8780, E-Mail: [email protected]

www.staff.city.ac.uk/~ra600/intro.htm

Engineering Drawing and Design - Lecture 7

ME 1110 – Engineering Practice 1

http://www.staff.city.ac.uk/%7Era600/PageME1110.htm



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Objectives for today

• To learn about fits and tolerances

• To learn how to define tolerance in order for parts to function correctly




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Example detailed drawing




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Tolerancing• Definition:

» Allowance for specific variation in the size and geometry of a part

• Why is tolerancing necessary?» It is impossible to manufacture a part to an exact size

or geometry» Since variation from the drawing is inevitable the

acceptable degree of variation must be specified» Large variation may affect the functionality of the part» Small variation will effect the cost of the part

requires precise manufacturing requires inspection and the rejection of parts




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Tolerance Declaration

Tolerance can be expressed in different ways:1. Direct tolerancing method (size)

» Limits specifying the allowed variation in each dimension (length, width, height, diameter, etc.) are given on the drawing

2. General tolerance note» Notes like “ALL DIMENSIONS HELD TO ±0.05”

3. Geometric tolerancing» Allows for specification of tolerance for the geometry of a part

separate from its size» GDT (Geometric Dimensioning and Tolerancing) uses special

symbols to control different geometric features of a part




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Direct Tolerancing method

(1) Limit Dimensioning

(2) Deviation tolerancing

unilateral bilateral

equal

unequal




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Tolerancing - Terminology

1 Basic Size:- From which limits are fixedDeviation:- Difference - between the

basic and allowed size2 Max Limit3 Min Limit

Upper and lower limit

Symmetric

Hole/shafttolerance

Deviation




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BS4500: ISO Units A system of 18 grades and tolerances related

to part size ranges (i.e. basic size range) These standard tolerances are related to the

zero line by a letter code i.e. deviation




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1 Basic Size - From which limits are fixedDeviation - Difference - The basic and the another size

2 Max Limit - Of size permissible3 Min Limit - Of size permissible4 Upper deviation - 1-2 5 Lower deviation - 1-36 Tolerance - 2-3 or 4-5

Tolerance and Fits - terminology

ZERO LINE4

53shaft2shaft

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4

5

3hole 2hole1

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To differentiate between holes and shafts, upper and lower case letters are used H – Holes; h - Shafts




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Fitting Two Parts

Part A

Tolerance of APart B

Tolerance of B

Fit: Clearance or Interference

Presenter

Presentation Notes

Instructor: The blue area represents the fit between A and B where the dark area represent the variance in the size of A and B




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Tolerance Terminology – continue

ZERO LINE4

53shaft2shaft

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4

5

3hole 2hole1

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1 Nominal Size– a general size, common fraction Basic Size – theoretical size from which limits are fixedActual Size – measured size

2,3 Limits – maximum and minimum permissible sizes4,5 Deviation – max. and min. difference from a nominal

size (1-2 or 1-3)6 Tolerance – total allowable variance in dimensions

(upper limit – lower limit or 2-3 or 4-5)




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Fits• Range of tightness between two mating parts• Types of fit

» Clearance fits– provides clearance between two mating parts.

» Interference fit– results in interference between mating parts

» Transition fits– results in neither of the above




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Shaft and Hole FitsClearance InterferenceTransition

Presenter

Presentation Notes

Instructor: Here is another way to look at how parts fit together. In this illustration, the beige area represents the variation in size of one part and it is easy to see that on the left side we have a clearance fit where the smallest hole is larger than the largest shaft. This is called a clearance fit. The opposite is true on the other end. This is a force fit, shrink fit or an interference fit. Note the small print at the bottom: Allowance always equals the smallest hole minus the largest shaft. When you have an interference fit the allowance is negative.




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Shaft and Hole FitsTransitionClearance Interference

Presenter

Presentation Notes

Instructor: This figure is slightly different than the previous one. Here they have put the smallest shaft on the left end and the largest shaft on the right side. The shaft on the left clears by .003 inch while the one on the right interferes by .002 inch.




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Preferred Hole Basis System of Fits

Standardised by BS4500: ISO Units and Fits

Presenter

Presentation Notes

Instructor: When working in the metric system they use a series of letters and numbers where the capital letters represent the hole and the lower case letters represent the shaft sizes.




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Basic Hole System or Hole Basis

Definition of the "Basic Hole System":» The "minimum size" of the hole is equal to

the "basic size" of the fit

Example: If the nominal size of a fit is 10 mm, then the minimum size of the hole in the system will be 10mm

Presenter

Presentation Notes

Instructor: There needs to be some order to doing tolerances and in this case the choice was to make the smallest hole the basic size. If the nominal size is given as a common fraction (1/2) then the basic size is .500 or .5000 depending on the type of fit.




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Fundamental deviations for shafts




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Fit Calculations Clearance = Hole – Shaft Cmax = Hmax–Smin

Cmin = Hmin –Smax

If:» Both Cmax and Cmin >0 - Clearance fit» Both Cmax and Cmin <0 - Interference fit» Cmax > 0 and Cmin < 0 - Transition fit

Allowance = Hmin – Smax = Cmin

System tolerance: TS = Cmax - Cmin = Σ Ti

Presenter

Presentation Notes

Instructor: Go through this table of equations with your class. These are the equations that they will use to complete today’s exercises.




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Example ISO Tolerance

Grades

Fundamental Deviations

0+0.025 40.025 40.000 -0.009 -0.025 39.991 39.975

Max Clearance

Min Clearance

Allowance

System Tol:

Cmax=0.050

Cmin=0.009

= Cmin

TS =0.041

Homework

Presenter

Presentation Notes

Instructor: Here is an example of looking at the tolerance of each part and the fit tolerance.




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Exercise DrE-5

• Groups of 5. Each group has one assembly with several parts.

• Measure parts in the assembly together.

• Each member of the group will have to do his/her own part.

• Make sketch - drawing with all required dimensions, tolerances and surface finish notes.

• This sketch has to be approved and as such used as the basis for CAD-1 exercise.

• 2 week exercise



Documents

Fits and Tolerances - City, University of Londonra600/ME1105/Lectures/ME1110-07.pdf · To learn about fits and tolerances ... 1 Nominal Size– a general size, ... Actual Size –